Field of the Invention
The present invention relates to a pellicle frame of a pellicle and the pellicle that are used as an extraneous-material cover for a photomask for use in lithography when a semiconductor integrated circuit such as an integrated circuit (IC) and large scale integration (LSI), or a liquid crystal display (LCD) is produced.
Description of the Related Art
In production of a semiconductor integrated circuit, such as IC and LSI, or a liquid crystal display, LCD, patterns are formed by photolithography involving an exposure process. If extraneous material is attached to an exposure plate (a photomask for use in lithography) used in this process, not only the patterns but also an image of the attached extraneous material is transferred by the exposure. This impairs the size, quality, and appearance of the patterns, thereby reducing production yield in the photolithography process.
In view of this problem, these operations are usually performed in a clean room. It is however difficult to always keep the exposure plate clean even in the clean room. Accordingly, an extraneous-material cover called a pellicle through which an exposure light readily passes is attached to the exposure plate. This method prevents direct attachment of extraneous material to the exposure plate and causes extraneous material to attach to a pellicle film. In this case, focus is adjusted to the pattern of the exposure plate in photolithography so that the transfer can be prevented from being affected by extraneous material on the pellicle film.
The pellicle is basically constructed of a pellicle film made of a material through which an exposure light readily passes, such as nitrocellulose, cellulose acetate or fluorine-based polymer, a pellicle frame made of stainless steel, polyethylene, or an aluminum alloy subjected to a black alumite process, an adhesive layer made of polybutene resin, polyvinyl acetate, acrylic resin, or silicone resin to attach the pellicle to the exposure plate, and a liner to protect the adhesive layer.
The pellicle film is stuck in such a way that a good solvent of the pellicle film is applied to the upper end surface of the pellicle frame, and the pellicle film is brought close contact with the pellicle frame, and then air-dried (See Patent Document 1), or in such a way as to use an adhesive such as acrylic resin, epoxy resin, or fluorine resin (See Patent Documents 2 and 3). The adhesive layer is formed on the lower end surface of the pellicle frame.
The reason why the aluminum alloy used for the pellicle frame is subjected to the black alumite process is to prevent the exposure light from being reflected by the pellicle frame and to facilitate inspection of extraneous material on the pellicle. In general, it is thus preferable to make the pellicle frame black.
The pellicle is mounted so as to cover a pattern region formed on a surface of the exposure plate. Since this pattern region covered by the pellicle is isolated from the exterior, the exposure plate can be prevented from directly contacting extraneous material such as dust produced in the exterior of the pellicle.
In recent years, the increasing scale of integration of LSI promotes shortening the wavelength of the source of exposure light. The mainstream source of the exposure light is thereby being shifted from G-line (436 nm) or I-line (365 nm) of a mercury lamp to a KrF excimer laser (248 nm) or an ArF excimer laser (193 nm).
The shortened wavelengths of the source of exposure light, however, increases the energy of the exposure light, thereby increasing the possibility of occurrence of extraneous material called haze on a photomask substrate due to reaction of gaseous substances present in an exposure atmosphere.
Accordingly countermeasures are taken: for example, gaseous substances in the clean room are reduced as much as possible, a photomask is rigorously cleaned, or a pellicle made of a material that produces less gas is used.
However, even though the photomask is cleaned and the pellicle made of a material that produces less gas is used, the occurrence of haze on the photomask substrate cannot be completely prevented.
An anodized coating formed on a surface of an aluminum alloy used for the pellicle frame, for example, contains acidic components such as sulfuric acid, nitric acid, and organic acid. When these acidic components are separated from the anodized coating and react in a closed space between the pellicle and the photomask under exposure environment, a sulfur compound such as an ammonium sulfate are produced. The haze occurs due to this produced sulfur compound attached to the photomask substrate. The occurrence of the haze causes reduction in yield of semiconductor production.
A known method to inhibit the occurrence of haze is to remove acidic components from the anodized coating by ultrasonic cleaning the anodized pellicle frame in pure water (See Patent Document 4). In addition, there are proposed methods in which a polymer coating is formed by electrodeposition instead of the anodized coating (See Patent Document 5), or metal plating is performed (See Patent Document 6), or a colorless aluminum alloy is used (See Patent Document 7).
When aluminum subjected to the plating process instead of the black alumite process as disclosed in Patent Document 6 or the colorless aluminum alloy as disclosed in Patent Document 7 is used for the pellicle frame, however, the visibility in extraneous-material inspection becomes worse. A pellicle frame coated with a polymer colored with black pigment such as an iron oxide, a titanium dioxide, or carbon black, as disclosed in Patent Document 5, is undesirable from the viewpoint of particle generation.